Refractory materials formed from mats or blankets of ceramic fibers are routinely used to line the interior of high temperature furnaces and other devices involving exposure to high temperature conditions. The insulating materials are typically formed from layers of fibrous refractory material that are often assembled into modules, which may have a cube-shape. The cube-shape is commonly utilized to facilitate post manufacturing shipping, handling and installation. The layers of fibrous material may be held together by a variety of methods and each layer is commonly composed of the same material throughout. As illustrated in FIG. 1, the layers are typically assembled in a side-by-side orientation such that each layer extends completely across from the hot face through to the cold face on the opposite side of the module. Such modules are commonly installed on an interior furnace wall such that the hot face is exposed to the interior of the furnace and the cold face is mounted against the furnace wall.
When a furnace is designed to operate at very high temperature, e.g., such that the furnace walls must be capable of withstanding temperatures in the range of 2000° F. up to 3000° F., the ceramic materials capable of withstanding such conditions can be quite expensive. In general, only the outer portion on the hot face of the module is actually exposed to these very high temperatures, since the insulating effect of the ceramic fiber material will allow a substantially lower temperature to be maintained on the cold face of the module against the furnace interior wall. In furnaces insulated on the interior with such insulating modules, it is quite common for the temperature on the furnace casing next to the cold face of the module to be maintained at a temperature that is substantially cooler, e.g., about 1,000° F. or more, cooler than the hot face of the module. If the module is composed of insulating layers of a single material that extend all the way from the hot face to the cold face of the module, this means that the expensive insulating fibrous refractory material necessary to withstand the very high temperatures on the hot face must be used throughout the module, a solution which is not cost effective.
Efforts have been made to design insulating modules that employ two types of ceramic materials, with an expensive ceramic material rated for very high temperatures on the hot face and a less expensive material with a lower temperature rating on the cold face. The approaches reported to date suffer from various disadvantages. For example, U.S. Pat. No. 4,379,382 describes a high temperature insulation module having one type of ceramic fiber mat on the hot face and a second fiber mat on its cold face. The two ceramic mats are held together by a planar support member positioned between the mats. The two ceramic mats are either bonded to the support member by layers of cement applied to the outer periphery of the support member or by means of pins or clips attaching a mat to the support member. Either method of attachment can be subject to failure under certain conditions as well as the support member, which is commonly formed from metal, being potentially subject to deterioration due to corrosion.